1. Field of the Invention
The present invention relates to a measurement control method of an injection molding machine that can be preferably used in performing measurements by screw rotation.
2. Description of the Relevant Art
In general, an injection cycle of an injection molding machine has a measurement process and an injection process, and during the measurement process, measurement is performed by rotating a screw wherein measurement control is performed so that measurement is ended when the screw is retracted to a preset measurement ending position. Although various types of controls are performed during the measurement process such as rate control, pressure control, and position control, improving control precision for a series of controls in the measurement process is extremely important for securing uniform injection molded product mass and obtaining an injection molded object of high quality, and various kinds of measurement control methods have conventionally been proposed.
For example, disclosed in Japanese Patent Publication No. 6 (1994)-61800 (Patent Document 1) is a measurement control method of an injection molding machine (measurement control device) which, being a control method for retracting a screw by rotating it by a preset number of screw rotations and stopping the screw at a preset measurement ending position, receives a screw position detected by a screw position detection means and a screw retraction rate detected by a screw rate detection means, calculates the number of screw rotations so as to stop the screw at the measurement ending position by a predetermined calculation formula, for sending out the calculated number of screw rotations as a rotation drive command. Further, disclosed in Japanese Laid Open Patent Publication No. 2004-154988 (Patent Document 2) is a measurement control method for an injection molding machine (measurement method) which, after a screw is retraced to a set screw position in the vicinity of the set measurement ending position, derives the screw rotation rate proportional to the position deviation between the set measurement ending position and the current screw retraction position, corrects the screw rotation rate based on the pressure deviation between the set resin pressure and the currently detected resin pressure, making it a screw rotation rate command to control the screw rotation rate.
However, the conventional measurement control method for an injection molding machine as described above had the following problems.
First, as in Patent Document 1, when adopting a control method that detects the screw position and the screw retraction rate and calculates the number of screw rotations to stop the screw at the measurement ending position while adopting the calculated number of screw rotations as a rotation drive command, the rotation rate of the screw near the measurement ending position infinitely approaches zero, thus requiring a significant amount of time until the screw arrives at the measurement ending position. Therefore, while it is advantageous in improving the control accuracy of the screw position, since the cycle time cannot be shortened, it is extremely disadvantageous in realizing a high-speed molding and limits in improvement of molding efficiency and mass productivity.
Further, as in Patent Document 2, when adopting a measurement control method which, after a screw is retracted to a set screw position in the vicinity of the set measurement ending position, derives the screw rotation rate proportional to the position deviation between the set measurement ending position and the current screw retraction position and corrects it based on the pressure deviation between the set resin pressure and the currently detected resin pressure to make it a screw rotation rate command, and control in the vicinity of the set measurement ending position becomes only position control, thereby fixing the control target. While it is advantageous in improving the control accuracy of the screw position, it introduces complications of control such as the necessity of controlling the screw rotation rate for controlling back pressure, making it difficult to secure responsiveness and stability in realizing the back pressure control within a section of minimal length.
Further, while it is required in measurement control that both screw rotation and retraction are reliably stopped at a measurement completion position (measurement ending position) for securing a high measurement precision, in the case of Patent Documents 1 and 2, no consideration is given to reliably stopping both screw rotation and retraction, thus they cannot deal with molding of, especially for instance, recent thin optical disks requiring a high degree of measurement precision.